1. Field of the Invention
This invention relates to ion trap devices and, more particularly, to such devices that are formed by out-of-plane assembly of micro-cavities on a semiconductor or dielectric wafer.
2. Discussion of the Related Art
Conventional ion traps enable ionized particles to be stored and the stored ionized particles to be separated according to the ratio (M/Q) of their mass (M) to their charge (O). Storing the ionized particles involves applying a time-varying voltage to the ion trap so that particles propagate along stable trajectories therein. Separating the ionized particles typically involves applying an additional time-varying voltage to the trap so that the stored particles are selectively ejected according to their M/Q ratios. The ability to eject particles according to their M/Q ratios enables the use of ion traps as mass spectrometers.
Exemplary ion traps are described, for example, by W. Paul et al. in U.S. Pat. No. 2,939,952 issued Jun. 7, 1960. One such ion trap, known as a quadrupole, is described by R. E. March in “Quadrupole Ion Trap Mass Spectrometer,” Encyclopedia of Analytical Chemistry, R. A. Meyers (Ed.), pp. 11848–11872, John Wiley & Sons, Ltd., Chichester (2000). Both of these documents are incorporated herein by reference.
FIG. 1 herein shows one type of quadrupole ion trap 10 that has an axially symmetric cavity 18 akin to that depicted in FIG. 2 of March. More specifically, the ion trap 10 includes metallic top and bottom end cap electrodes 12–13 and a metallic central ring-shaped electrode 14 that is located between the end cap electrodes 12–13. Points on inner surfaces 15–17 of the electrodes 12–14 have transverse radial coordinates r and axial coordinates z. These coordinates satisfy hyperbolic equations; i.e., r2/r02−z2/z02=+1 for the central ring-shaped electrode 14 and r2/r02−z2/z02=−1 for the end cap electrodes 12–13. Here, 2r0 and 2z0 are, respectively, the minimum transverse diameter and the minimum vertical height of the trapping cavity 18 that is formed by the inner surfaces 15–17. Typical trapping cavities 18 have a shape ratio, r0/z0, that satisfies: (r0/z0)2≈2, but the ratio may be smaller to compensate for the finite size of the electrodes 12–14. Typical cavities 18 have a size that is described by a value of r0 in the approximate range of about 0.707 centimeters (cm) to about 1.0 cm. We refer to cavities of this approximate size as macro-cavities.
For the above-described electrode and macro-cavity shapes, electrodes 12–14 produce an electric field with a quadrupole distribution inside trapping cavity 18. One way to produce such an electric field involves grounding the end cap electrodes 12–13 and applying a radio frequency (RF) voltage to the central ring-shaped electrode 14. In an RF electric field having a quadrupole distribution, ionized particles with small Q/M ratios will propagate along stable trajectories. To store particles in the trapping cavity 18, the cavity 18 is voltage-biased as described above, and ionized particles are introduced into the trapping cavity 18 via ion generator 19.1 coupled to entrance port 19.2 in top end cap electrode 12. During the introduction of the ionized particles, the trapping cavity 18 is maintained with a low background pressure; e.g., about 10−3 Torr of helium (He) gas. Then, collisions between the background He atoms and ionized particles lower the particles' momenta, thereby enabling trapping of such particles in the central region of the trapping cavity 18. To eject the trapped particles from the cavity 18, a small RF voltage may be applied to the bottom end cap 13 while ramping the small voltage so that stored particles are ejected through exit orifice 19.4 selectively according to their M/Q ratios. The ejected ions are then incident on a utilization device 19.3 (e.g., an ion collector), which is coupled to orifice 19.4.
For quadrupole ion trap 10, machining techniques are available for fabricating hyperbolic-shaped electrodes 12–14 out of base pieces of metal. Unfortunately, such machining techniques are often complex and costly due to the need for the hyperbolic-shaped inner surfaces 15–17. For that reason, other types of ion traps are desirable.
A second type of ion trap has a trapping macro-cavity with a right circularly cylindrical shape. This trapping cavity is also formed by inner surfaces of two end cap electrodes and a central ring-shaped electrode located between the end cap electrodes. Here, the end cap electrodes have flat disk-shaped inner surfaces, and the ring-shaped electrode has a circularly cylindrical inner surface. For such a trapping cavity, applying a voltage to the central ring-shaped electrode while grounding the two end cap electrodes will create an electric field that does not have a pure quadrupole distribution. Nevertheless, a suitable choice of the trapping cavity's height-to-diameter ratio will reduce the magnitude of higher multipole contributions to the created electric field distribution. In particular, if the height-to-diameter ratio is between about 0.83 and 1.00, the octapole contribution to the field distribution is small; e.g., this contribution vanishes if the ratio is about 0.897. For such values of this shape ratio, the effects of higher multipole distribution are often small enough so that the macro-cavity is able to trap and store ionized particles. See, for example, J. M. Ramsey et al., U.S. Pat. No. 6,469,298 issued on Nov. 22, 2002, which is incorporated herein by reference.
For this second type of ion trap, standard machining techniques are available to fabricate the electrodes from metal base pieces, because the electrodes have simple surfaces rather than the complex hyperbolic surfaces of the electrodes 12–14 of FIG. 1. For this reason, fabrication of this second type of ion trap is usually less complex and less expensive than is fabrication of quadrupole ion traps whose electrodes have hyperbolic-shaped inner surfaces.
Nevertheless, the metallic components of such ion traps are expensive to manufacture and assemble. Moreover, these metallic components cause equipment in which they are incorporated to be large and bulky. The latter property has limited the widespread application and deployment of these ion traps in equipment such as mass spectrometers and shift registers.
Thus, a need remains in the art for a micro-miniature ion trap that can be inexpensively and readily implemented without reliance on the metallic components common to the prior art. In particular, there is a need for such an ion trap that has a micro-cavity that can be readily and inexpensively fabricated and assembled.